dsp: use default bit-width when start VoLTE call in power mode

VoLTE in power mode doesn't support to configure bit-width of
AFE port with 24bit.

Fix it by use default bit-width when start VoLTE in power mode.

Change-Id: Ic429ea317c71c9a4436705663185fc69cb885166
Signed-off-by: Kunlei Zhang <kunleiz@codeaurora.org>